The present invention relates generally to the field of semiconductor processing equipment and more specifically to a method and apparatus for eliminating contaminants and residues from inside a vacuum exhaust line connected to a processing chamber and to a method and apparatus for reducing perfluorocompound (PFC) gas emissions from a processing chamber.
During chemical vapor deposition (CVD) processing, deposition gases are released inside a processing chamber to form a thin film layer on the surface of a substrate being processed. Unwanted deposition on areas such as the walls of the processing chamber also occurs during such CVD processes. Because the residence time in the chamber of individual molecules in these deposition gases is relatively short, however, only a small portion of the molecules released into the chamber are consumed in the deposition process and deposited on either the wafer or chamber walls.
The unconsumed gas molecules are pumped out of the chamber along with partially reacted compounds and reaction byproducts through a vacuum line that is commonly referred to as the "foreline." Many of the compounds in this exhausted gas are still in highly reactive states and/or contain residues or particulate matter that can form unwanted deposits in the foreline. Given time, this deposition build-up of powdery residue and/or particulate matter presents a problem. First, the matter is often a pyrophoric substance that may present problems when the vacuum seal is broken and the foreline is exposed to ambient conditions during standard, periodic cleaning operations. Second, if enough of the deposition material builds-up in the foreline, the foreline and/or its associated vacuum pump may clog if it is not appropriately cleaned. Even when periodically cleaned, matter build-up interferes with normal operation of the vacuum pump and can drastically shorten the useful life of the pump. Also, the solid matter may backwash from the foreline into the processing chamber and contaminate processing steps adversely effecting wafer yield.
To avoid these problems, the inside surface of the foreline is regularly cleaned to remove the deposited material. This procedure is performed during a standard chamber clean operation that is employed to remove unwanted deposition material from the chamber walls and similar areas of the processing chamber. Common chamber cleaning techniques include the use of an etching gas, such as fluorine, to remove the deposited material from the chamber walls and other areas. The etching gas is introduced into the chamber and a plasma is formed so that the etching gas reacts with and removes the deposited material from the chamber walls. Such cleaning procedures are commonly performed between deposition steps for every wafer or every N wafers.
Removal of deposition material from chamber walls is relatively straightforward in that the plasma is created within the chamber in an area proximate to the deposited material. Removal of deposition material from the foreline is more difficult because the foreline is downstream from the processing chamber. In a fixed time period, most points within the processing chamber come in contact with more of the etchant fluorine atoms than do points within the foreline. Thus, in a fixed time period, the chamber may be adequately cleaned by the clean process while residue and similar deposits remain in the foreline.
To attempt to adequately clean the foreline, the duration of the clean operation must be increased. Increasing the length of the clean operation, however, is undesirable because it adversely effects wafer throughput. Also, such residue buildup can be cleaned only to the extent that reactants from the clean step are exhausted into the foreline in a state that they may react with the residue in the foreline. In some systems and applications, the lifetime of the exhausted reactants is not sufficient to reach the end or even middle portions of the foreline. In these systems and applications, residue build-up is even more of a concern. Accordingly, there is a need for an apparatus for efficiently and thoroughly cleaning the foreline in a semiconductor processing system and a method of doing the same.
One approach that has been employed to clean the foreline relies on a scrubbing system that uses plasma enhanced CVD techniques to extract reactive components in the exhaust gas as film deposits on electrode surfaces. The scrubbing system is designed to maximize the removal of reactants as a solid film and uses large surface area spiral electrodes. The spiral electrodes are contained within a removable canister that is positioned near the end of the foreline between the blower pump and mechanical pump. After a sufficient amount of solid waste has built up on the electrodes, the canisters may be removed for disposal and replacement.
Problems exist in this prior art method in that the system relies on the large surface area of the electrodes to provide an area for deposited solid matter to collect. To accommodate the large surface area of the electrodes, the system is necessarily large and bulky. Furthermore, extra expenses are incurred in the operation of this prior art scrubber system since the removable canister is a disposable product that must be replaced and properly disposed. Also, the scrubbing system is located downstream from a beginning portion of the vacuum foreline and thus does not ensure removal of powdery material or particulate matter that builds-up in this portion of the line.
From the above it can be seen that an improved method and apparatus for cleaning a foreline is desirable.
Another issue of concern in CVD and other substrate processing apparatus relates to the types of gases and byproducts exhausted from the processing chamber through the foreline. For example, because dissociation of gas within the cleaning plasma is not complete (in some applications only 10% of the introduced gas molecules are dissociated), and the residence time in the chamber of individual molecules in the cleaning gas is relatively short, only a small portion of the molecules released into the chamber react with the deposited material. The gas molecules that do not take part in an etchant reaction are pumped out of the chamber along with the etched away material and reaction byproducts through a vacuum line that is commonly referred to as the "foreline." The exhausted gases are emission byproducts of the semiconductor process.
Many of the fluorine containing gases employed in the semiconductor industry as cleaning etchant gases are referred to as perfluorocompounds or "PFC's" for short. Some of the more commonly used PFC's include CF.sub.4, C.sub.2 F.sub.6, NF.sub.3 and SF.sub.6 or similar gases. These gases are known to have a long lifetime (up to 50,000 years for CF.sub.4), and it is also believed that they have a large global warming potential. Thus, their release into the atmosphere is potentially damaging and is becoming the subject of government and other regulations. Accordingly, it is important to reduce PFC emissions from semiconductor processing equipment such as CVD reaction chambers.